1. Field of the Invention
This invention relates to viscous differentials and particularly to a viscous differential which has provision for releasable mounting to the axles of a vehicle.
2. Description of the Prior Art
Differentials are provided in vehicle drive trains in order to permit drive wheels to turn at different speeds. This is necessary because if each of the drive wheels were to turn at the same rate, scuffing on the road surface would result during common maneuvers, such as making a turn. Attendant to this capacity of a differential, a drive wheel which encounters a slippery road surface will receive the majority of torque of the drive train while the drive wheel which has good traction will not. This circumstance, frequently encountered on icy and wet roads, can lead to situations in which the vehicle becomes stuck. The use of four-wheel drive systems has the advantage that all four wheels will receive torque from the drive train. However, four-wheel drive systems must allow for individual wheel rotation if this system is to used for other than only off-road purposes.
Accordingly, what is required is a limited slip differential that can allow for limited differential drive wheels to rotate at different rates during normal driving conditions and also deliver torque to the drive wheel or drive wheels which have the best traction even though one or more of the other drive wheels have less traction due to a slippery road surface.
One means to accomplish limited slip is to mechanically link the two drive wheels in a two-wheel drive system or all four wheels in a four-wheel drive system, effectively bypassing the differential. In a turn, such a vehicle drive train will result in tire scuffing. Consequently, this drive train is best suited for off-road vehicle operation where terrain conditions make this a preferred system.
In the prior art, it has been found that control of the coupling between the drive wheels to maximize friction with the road surface and yet permit differentiation between the drive wheels is best accomplished by selectively coupling the drive wheels so that part-time four-wheel drive is available when needed. These systems utilize either mechanical couplings or viscous couplings to accomplish control of wheel differentiation.
In mechanical couplings, a control is used to engage and disengage a direct mechanical connection between the drive wheels. Examples of these systems are as follows: U.S. Pat. No. 4,609,064 to Suzuki discloses a four-wheel drive system which electronically senses when the vehicle is making a turn and thereupon changes the drive train from four-wheel drive to differential two-wheel drive. U.S. Pat. No. 4,058,027 to Webb and U.S. Pat. No. 4,286,686 to Franke disclose a friction plate coupling between the drive wheels which is actuated by a hydraulic clutch. U.S. Pat. No. 4,545,456 to Lake discloses a four-wheel drive system which has a selectable chain drive member that uses cams to engage a spline connection to the front wheels. U.S. Pat. No. 4,524,640 to Neumann et al discloses a differential having hold out rings which keep an overrunning axle (as would occur during a turn) disengaged from a spider drive member. Finally, U.S. Pat. No. 3,987,689 to Engle discloses a limited slip differential having a rotatable casing and an actuator for engaging friction clutch plates when a predetermined level of relative rotation is present.
In viscous couplings, interleave plates within a viscous fluid are used to selectively interconnect the drive wheels. The controls therefor may be either manually selective or automatically selective. U.S. Pat. No. 2,949,046 to Critelli is exemplary of a viscous coupling differential, wherein a housing contains a planetary and orbit gear differential, a first series of annular plates attached to one drive wheel axle, a second set of annular plates, interleaved with the first set of annular plates, attached to the other drive wheel axle and a high viscous fluid which fills the housing. When one of the drive wheels encounters a slippery road surface which results in wheels slipping, relative rotation among the annular plates causes viscosity of the fluid to increase, transmitting torque to the drive wheel axle which has the best traction.
In the prior art, many improvements have been made to the basic type of viscous differential that is represented by Critelli. Examples of these improvements are as follows: U.S. Pat. No. 4,605,087 to Ashauer et al discloses a viscous coupling between the front and rear wheels of a vehicle, and, in addition, a coupling device which disengages the viscous coupling to the rear wheels during severe braking. U.S. Pat. No. 4,562,897 to Rennecker discloses a viscous coupling between the front and rear wheels of a vehicle which is selectable between a disengaged setting, an engaged setting, and a disengaged setting having in addition direct mechanical connection between the front and rear wheels. U.S. Pat. No. 4,601,359 to Weismann et al discloses an on demand front wheel drive which incorporates two differentials, one for the front wheels and one for the front to the rear wheels. The front to rear differential includes a viscous coupling which delivers torque to the set of drive wheels having the best road traction. European Patent 0,167,312 to Diggins discloses a viscous coupling between three coaxial rotatable members, where two of the three coaxial members have annular plates that interleave annular plates of the third coaxial member.
Particularly relevant to the present invention are the following patents. U.S. Pat. No. 4,548,096 to Giocastro et al discloses a fluid torque converter located at the wheel hub having a demountable axle using a key and nut arrangement. U.S. Pat. No. 4,650,028 to Eastman et al discloses a part-time on demand four-wheel drive system wherein a single viscous coupling apparatus is incorporated in the vehicle rear axle serving as both an interaxle differential, dividing the engine torque between the front and rear axles, as well a limited slip rear differential. The viscous coupling has an outer drum circumferentially surrounding a pair of left and right drums, each respectively connected to the left and right rear axles. Annular plates connected to the outer drum are interleaved with annular plates connected to the left and right drums. A mechanical linkage to the front axle is provided by gearing between a driving bevel gear and a crown gear connected to the outer drum. When a slippage condition exists in the front drive wheels, relative rotation between the annular plates in the viscous coupling will transfer torque to the rear wheels. Further, relative rotation between the rear wheels will adjust torque between the rear wheels to the wheel having the best traction.
There remains in the prior art a need to devise an on demand limited slip differential which incorporates both an interaxle and a limited slip differential that has provision for releasable connection to the drive wheel axles.